Voltage threshold ablation method and apparatus

ABSTRACT

A method and apparatus for treating tissue using an electrosurgical system. The system includes an electrosurgical system having an RF generator, a treatment electrode electrically coupled to the RF generator and positioned in contact with target tissue to be treated, and a spark gap switch positioned between the RF generator and the target tissue. The spark gap includes a threshold voltage and is configured to prevent conduction of current from the RF generator to the tissue until the voltage across the spark gap reaches the threshold voltage. 
     The method includes the steps of using the RF generator to apply a voltage across the spark gap switch, the spark gap switch causing conduction of current from the RF generator to the target tissue once the voltage across the spark gap reaches the threshold voltage.

FIELD OF THE INVENTION

The present invention relates to the field of electrosurgery, and moreparticularly to methods for ablating, cauterizing and/or coagulatingbody tissue using radio frequency energy.

BACKGROUND OF THE INVENTION

Radio frequency ablation is a method by which body tissue is destroyedby passing radio frequency current into the tissue. Some RF ablationprocedures rely on application of high currents and low voltages to thebody tissue, resulting in resistive heating of the tissue whichultimately destroys the tissue. These techniques suffer from thedrawback that the heat generated at the tissue can penetrate deeply,making the depth of ablation difficult to predict and control. Thisprocedure is thus disadvantageous in applications in which only a finelayer of tissue is to be ablated, or in areas of the body such as theheart or near the spinal cord where resistive heating can result inundesirable collateral damage to critical tissues and/or organs.

It is thus desirable to ablate such sensitive areas using high voltagesand low currents, thus minimizing the amount of current applied to bodytissue.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for treating tissueusing an electrosurgical system. The system includes an electrosurgicalsystem having an RF generator, a treatment electrode electricallycoupled to the RF generator and positioned in contact with target tissueto be treated, and a spark gap switch positioned between the RFgenerator and the target tissue. The spark gap includes a thresholdvoltage and is configured to prevent conduction of current from the RFgenerator to the tissue until the voltage across the spark gap reachesthe threshold voltage.

A method according to the present invention includes the steps of usingthe RF generator to apply a voltage across the spark gap switch, thespark gap switch causing conduction of current from the RF generator tothe target tissue once the voltage across the spark gap reaches thethreshold voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side elevation view of a first embodiment ofan ablation device utilizing principles of the present invention.

FIG. 2 is an end view showing the distal end of the device of FIG. 1.

FIG. 3 is a graphical representation of voltage output from an RFgenerator over time.

FIG. 4A is a graphical representation of voltage potential across a bodytissue load, from an ablation device utilizing voltage thresholdablation techniques as described herein.

FIG. 4B is a graphical representation of voltage potential across a bodytissue load, from an ablation device utilizing voltage thresholdablation techniques as described herein and further utilizing techniquesdescribed herein for decreasing the slope of the trailing edge of thewaveform.

FIGS. 5A through 5D are a series of cross-sectional side elevation viewsof the ablation device of FIG. 1, schematically illustrating use of thedevice to ablate tissue.

FIG. 6A is a cross-sectional side view of a second embodiment of anablation device utilizing principles of the present invention.

FIG. 6B is an end view showing the distal end of the device of FIG. 6A.

FIGS. 7A and 7B are cross-sectional side elevation view of a thirdembodiment of an ablation device utilizing principles of the presentinvention. In FIG. 7A, the device is shown in a contracted position andin FIG. 7B the device is shown in an expanded position.

FIG. 8A is a perspective view of a fourth embodiment of an ablationdevice utilizing principles of the present invention.

FIG. 8B is a cross-sectional side elevation view of the ablation deviceof FIG. 8A.

FIG. 9A is a perspective view of a fifth embodiment of an ablationdevice utilizing principles of the present invention.

FIG. 9B is a cross-sectional side elevation view of the ablation deviceof FIG. 9A.

FIG. 10 is a cross-sectional side elevation view of a sixth ablationdevice utilizing principles of the present invention.

FIG. 11A is a perspective view of a seventh embodiment of an ablationdevice utilizing principles of the present invention.

FIG. 11B is a cross-sectional side elevation view of the ablation deviceof FIG. 11A.

FIG. 11C is a cross-sectional end view of the ablation device of FIG.11A.

FIG. 12A is a perspective view of an eighth embodiment of an ablationdevice utilizing principles of the present invention.

FIG. 12B is a cross-sectional side elevation view of the ablation deviceof FIG. 12A.

FIG. 13A is a cross-sectional side elevation view of a ninth embodimentof an ablation device utilizing principles of the present invention.

FIG. 13B is a cross-sectional end view of the ablation device of FIG.13A, taken along the plane designated 13B—13B in FIG. 13A.

FIG. 14A is a cross-sectional side elevation view of a tenth embodimentof an ablation device utilizing principles of the present invention.

FIG. 14B is a front end view of the grid utilized in the embodiment ofFIG. 14A.

FIG. 15A is a cross-sectional side elevation view of an eleventhembodiment.

FIG. 15B is a cross-sectional end view of the eleventh embodiment takenalong the plane designated 15B—15B in FIG. 15A.

FIG. 15C is a schematic illustration of a variation of the eleventhembodiment, in which the mixture of gases used in the reservoir may beadjusted so as to change the threshold voltage.

FIGS. 16A-16D are a series of drawings illustrating use of the eleventhembodiment.

FIG. 17 is a series of plots graphically illustrating the impact ofargon flow on the ablation device output at the body tissue/fluid load.

FIG. 18 is a series of plots graphically illustrating the impact ofelectrode spacing on the ablation device output at the body tissue/fluidload.

FIG. 19 is a schematic illustration of a twelfth embodiment of a systemutilizing principles of the present invention, in which a spark gapspacing may be selected so as to pre-select a threshold voltage.

DETAILED DESCRIPTION

Several embodiments of ablation systems useful for practicing a voltagethreshold ablation method utilizing principles of the present inventionare shown in the drawings. Generally speaking, each of these systemsutilizes a switching means that prevents current flow into body untilthe voltage across the switching means reaches a predetermined thresholdpotential. By preventing current flow into tissue until a high thresholdvoltage is reached, the invention minimizes collateral tissue damagethat can occur when a large amount of current is applied to the tissue.The switching means may take a variety of forms, including but notlimited to an encapsulated or circulated volume of argon or otherfluid/gas that will only conduct ablation energy from an intermediateelectrode to the ablation electrodes once it has been transformed to aplasma by being raised to a threshold voltage.

The embodiments described herein utilize a spark gap switch forpreventing conduction of energy to the tissue until the voltagepotential applied by the RF generator reaches a threshold voltage. In apreferred form of the apparatus, the spark gap switch includes a volumeof fluid/gas to conduct ablation energy across the spark gap, typicallyfrom an intermediate electrode to an ablation electrode. The fluid/gasused for this purpose is one that will not conduct until it has beentransformed to conductive plasma by having been raised to a thresholdvoltage. The threshold voltage of the fluid/gas will vary withvariations in a number of conditions, including fluid/gas pressure,distance across the spark gap (e.g. between an electrode on one side ofthe spark gap and an electrode on the other side of the spark gap), andwith the rate at which the fluid/gas flows within the spark gap—ifflowing fluid/gas is used. As will be seen in some of the embodiments,the threshold voltage may be adjusted in some embodiments by changingany or all of these conditions.

A first embodiment of an ablation device 10 utilizing principles of thepresent invention is shown in FIGS. 1-2. Device 10 includes a housing 12formed of an insulating material such as glass, ceramic, siliciumoxid,PTFE or other material having a high melting temperature. At the distalend 13 of the housing 12 is a sealed reservoir 20. An internal electrode22 is disposed within the sealed reservoir 20. Electrode 22 iselectrically coupled to a conductor 24 that extends through the housingbody. Conductor 24 is coupled to an RF generator 28 which may be aconventional RF generator used for medical ablation, such as the ModelForce 2 RF Generator manufactured by Valley Lab. A return electrode 30is disposed on the exterior surface of the housing 12 and is alsoelectrically coupled to RF generator 28.

A plurality of ablation electrodes 32 a-c are located on the distal endof the housing 12. Ablation electrodes 32 a-c may be formed of tungstenor any conductive material which performs well when exposed to hightemperatures. In an alternative embodiment, there may be only oneablation electrode 32, or a different electrode configuration. A portionof each ablation electrode 32 a-c is exposed to the interior ofreservoir 20. Electrodes 22 and 32 a-c, and corresponding electrodes inalternate embodiments, may also be referred to herein as spark gapelectrodes.

FIGS. 5A through 5D illustrate the method of using the embodiment ofFIG. 1. Referring to FIG. 5A, prior to use the reservoir 20 is filledwith a fluid or gas. Preferably, an inert gas such as argon gas or asimilar gas such as Neon, Xenon, or Helium is utilized to preventcorrosion of the electrodes, although other fluids/gases could beutilized so long as the electrodes and other components wereappropriately protected from corrosion. For convenience only, theembodiments utilizing such a fluid/gas will be described as being usedwith the preferred gas, which is argon.

It should be noted that while the method of FIGS. 5A-5D is mostpreferably practiced with a sealed volume of gas within the reservoir20, a circulating flow of gas using a system of lumens in the housingbody may alternatively be used. A system utilizing a circulating gasflow is described in connection with FIGS. 15A-15B.

The distal end of the device 10 is placed against body tissue to beablated, such that some of the electrodes 32 a, 32 b contact the tissueT. In most instances, others of the electrodes 32 c are disposed withinbody fluids F. The RF generator 28 (FIG. 1) is powered on and graduallybuilds-up the voltage potential between electrode 22 and electrodes 32a-32 c.

Despite the voltage potential between the internal electrode 22 andablation electrodes 32 a-c, there initially is no conduction of currentbetween them. This is because the argon gas will not conduct currentwhen it is in a gas phase. In order to conduct, high voltages must beapplied through the argon gas to create a spark to ionize the argon andbring it into the conductive plasma phase. Later in this descriptionthese voltages may also be referred to as “initiating voltages” sincethey are the voltages at which conduction is initiated.

The threshold voltage at which the argon will begin to immediatelyconduct is dependent on the pressure of the argon gas and the distancebetween electrode 22 and surface electrodes 32 a-32 c.

Assume P1 is the initial pressure of the argon gas within reservoir 20.If, at pressure P1, a voltage of V1 is required to ignite plasma withinthe argon gas, then a voltage of V>V1 must be applied to electrode 22 toignite the plasma and to thus begin conduction of current from electrode22 to ablation electrodes 32 a-32 c.

Thus, no conduction to electrodes 32 a-32 c (and thus into the tissue)will occur until the voltage potential between electrode 22 and ablationelectrodes 32 a-32 c reaches voltage V. Since no current flows into thetissue during the time when the RF generator is increasing its outputvoltage towards the voltage threshold, there is minimal resistiveheating of the electrodes 32 a-32 c and body tissue. Thus, this methodrelies on the threshold voltage of the argon (i.e. the voltage at whicha plasma is ignited) to prevent overheating of the ablation electrodes32 a, 32 b and to thus prevent tissue from sticking to the electrodes.

The voltage applied by the RF generator to electrode 22 cycles between+V and −V throughout the ablation procedure. However, as the processcontinues, the temperature of the tip of the device begins to increase,causing the temperature within the reservoir and thus the pressure ofthe argon to increase. As the gas pressure increases, the voltage neededto ignite the plasma also increases. Eventually, increases intemperature and thus pressure will cause the voltage threshold needed toignite the plasma to increase above V. When this occurs, flow of currentto the ablation electrodes will stop (FIG. 5D) until the argontemperature and pressure decrease to a point where the voltage requiredfor plasma ignition is at or below V. Initial gas pressure P1 and thevoltage V are thus selected such that current flow will terminate inthis manner when the electrode temperature is reaching a point at whichtissue will stick to the electrodes and/or char the tissue. This allowsthe tip temperature of the device to be controlled by selecting theinitial gas pressure and the maximum treatment voltage.

The effect of utilizing a minimum voltage limit on the potential appliedto the tissue is illustrated graphically in FIGS. 3 and 4A. FIG. 3 showsRF generator voltage output V_(RF) over time, and FIG. 4A shows theablation potential V_(A) between internal electrode 22 and body tissue.As can be seen, V_(A) remains at 0V until the RF generator output V_(RF)reaches the device's voltage threshold V_(T), at which time V_(A) risesimmediately to the threshold voltage level. Ablation voltage V_(A)remains approximately equivalent to the RF generator output until the RFgenerator output reaches 0V. V_(A) remains at 0V until the negativehalf-cycle of the RF generator output falls below (−V_(T)), at whichtime the potential between electrode 22 and the tissue drops immediatelyto (−V_(T)), and so on. Because there is no conduction to the tissueduring the time that the RF generator output is approaching the voltagethreshold, there is little conduction to the tissue during low voltage(and high current) phases of the RF generator output. This minimizescollateral tissue damages that would otherwise be caused by resistiveheating.

It is further desirable to eliminate the sinusoidal trailing end of thewaveform as an additional means of preventing application of lowvoltage/high current to the tissue and thus eliminating collateraltissue damage. Additional features are described below with respectFIGS. 14A-18. These additional features allow this trailing edge to beclipped and thus produce a waveform measured at the electrode/tissueinterface approximating that shown in FIG. 4B.

Another phenomenon occurs between the electrodes 32 a-32 c and thetissue, which further helps to keep the electrodes sufficiently cool asto avoid sticking. This phenomenon is best described with reference toFIGS. 5A through 5D. As mentioned, in most cases some of the electrodessuch as electrode 32 c will be in contact with body fluid while others(e.g. 32 a-b) are in contact with tissue. Since the impedance of bodyfluid F is low relative to the impedance of tissue T, current willinitially flow through the plasma to electrode 32 c and into the bodyfluid to return electrode 30, rather than flowing to the electrodes 32,32 b that contact tissue T. This plasma conduction is represented by anarrow in FIG. 5A.

Resistive heating of electrode 32 c causes the temperature of body fluidF to increase. Eventually, the body fluid F reaches a boiling phase anda resistive gas/steam bubble G will form at electrode 32 c. Steam bubbleG increases the distance between electrode 22 and body fluid F fromdistance D1 to distance D2 as shown in FIG. 5B. The voltage at which theargon will sustain conductive plasma is dependent in part on thedistance between electrode 22 and the body fluid F. If the potentialbetween electrode 22 and body fluid F is sufficient to maintain a plasmain the argon even after the bubble G has expanded, energy will continueto conduct through the argon to electrode 32 c, and sparking will occurthrough bubble G between electrode 32 c and the body fluid F.

Continued heating of body fluid F causes gas/steam bubble G to furtherexpand. Eventually the size of bubble G is large enough to increase thedistance between electrode 22 and fluid F to be great enough that thepotential between them is insufficient to sustain the plasma and tocontinue the sparking across the bubble G. Thus, the plasma betweenelectrodes 22 and 32 c dies, causing sparking to discontinue and causingthe current to divert to electrodes 32 a, 32 b into body tissue T,causing ablation to occur. See FIG. 5C. A gas/steam insulating layer Lwill eventually form in the region surrounding the electrodes 32 a, 32b. By this time, gas/steam bubble G around electrode 32 c may havedissipated, and the high resistance of the layer L will cause thecurrent to divert once again into body fluid F via electrode 32 c ratherthan through electrodes 32 a, 32 b. This process may repeat many timesduring the ablation procedure.

A second embodiment of an ablation device 110 is shown in FIGS. 6A and6B. The second embodiment operates in a manner similar to the firstembodiment, but it includes structural features that allow the thresholdvoltage of the argon to be pre-selected. Certain body tissues requirehigher voltages in order for ablation to be achieved. This embodimentallows the user to select the desired ablation voltage and to have thesystem prevent current conduction until the pre-selected voltages arereached. Thus, there is no passage of current to the tissue until thedesired ablation voltage is reached, and so there is no unnecessaryresistive tissue heating during the rise-time of the voltage.

As discussed previously, the voltage threshold of the argon varies withthe argon pressure in reservoir 120 and with the distance d across thespark gap, which in this embodiment is the distance extending betweenelectrode 122 and ablation electrodes 132 a-c. The second embodimentallows the argon pressure and/or the distance d to be varied so as toallow the voltage threshold of the argon to be pre-selected to beequivalent to the desired ablation voltage for the target tissue. Inother words, if a treatment voltage of 200V is desired, the user canconfigure the second embodiment such that that voltage will be thethreshold voltage for the argon. Treatment voltages in the range of 50Vto 10,000V, and most preferably 200V-500V, may be utilized.

Referring to FIG. 6A, device 110 includes a housing 112 formed of aninsulating material such as glass, ceramic, siliciumoxid, PTFE or otherhigh melting temperature material. A reservoir 120 housing a volume ofargon gas is located in the housing's distal tip. A plunger 121 isdisposed within the housing 112 and includes a wall 123. The plunger ismoveable to move the wall proximally and distally between positions 121Aand 121B to change the volume of reservoir 120. Plunger wall 123 issealable against the interior wall of housing 112 so as to preventleakage of the argon gas.

An elongate rod 126 extends through an opening (not shown) in plungerwall 123 and is fixed to the wall 123 such that the rod and wall canmove as a single component. Rod 126 extends to the proximal end of thedevice 110 and thus may serve as the handle used to move the plunger 121during use.

Internal electrode 122 is positioned within the reservoir 120 and ismounted to the distal end of rod 126 such that movement of the plunger121 results in corresponding movement of the electrode 122. Electrode122 is electrically coupled to a conductor 124 that extends through rod126 and that is electrically coupled to RF generator 128. Rod 126preferably serves as the insulator for conductor 124 and as such shouldbe formed of an insulating material.

A return electrode 130 is disposed on the exterior surface of thehousing 112 and is also electrically coupled to RF generator 128. Aplurality of ablation electrodes 132 a, 132 b etc. are positioned on thedistal end of the housing 112.

Operation of the embodiment of FIGS. 6A-6B is similar to that describedwith respect to FIGS. 5A-5B, and so most of that description will not berepeated. Operation differs in that use of the second embodimentincludes the preliminary step of moving rod 126 proximally or distallyto place plunger wall 123 and electrode 122 into positions that willyield a desired voltage threshold for the argon gas. Moving the plungerin a distal direction (towards the electrodes 132 a-c) will decrease thevolume of the reservoir and accordingly will increase the pressure ofthe argon within the reservoir and vice versa. Increases in argonpressure result in increased voltage threshold, while decreases in argonpressure result in decreased voltage threshold.

Moving the plunger 126 will also increase or decrease the distance dbetween electrode 122 and electrodes 132 a-c. Increases in the distanced increase the voltage threshold and vice versa.

The rod 126 preferably is marked with calibrations showing the voltagethreshold that would be established using each position of the plunger.This will allow the user to move the rod 126 inwardly (to increase argonpressure but decrease distance d) or outwardly (to decrease argonpressure but increase distance d) to a position that will give athreshold voltage corresponding to the voltage desired to be applied tothe tissue to be ablated. Because the argon will not ignite into aplasma until the threshold voltage is reached, current will not flow tothe electrodes 132 a, 132 b etc. until the pre-selected thresholdvoltage is reached. Thus, there is no unnecessary resistive tissueheating during the rise-time of the voltage.

Alternatively, the FIG. 6A embodiment may be configured such thatplunger 121 and rod 126 may be moved independently of one another, sothat argon pressure and the distance d may be adjusted independently ofone another. Thus, if an increase in voltage threshold is desired,plunger wall 123 may be moved distally to increase argon pressure, orrod 126 may be moved proximally to increase the separation distancebetween electrode 122 and 132 a-c. Likewise, a decrease in voltagethreshold may be achieved by moving plunger wall 123 proximally todecrease argon pressure, or by moving rod 126 distally to decrease theseparation distance d. If such a modification to the FIG. 6A wasemployed, a separate actuator would be attached to plunger 121 to allowthe user to move the wall 123, and the plunger 126 would be slidablerelative to the opening in the wall 123 through which it extends.

During use of the embodiment of FIGS. 6A and 6B, it may be desirable tomaintain a constant argon pressure despite increases in temperature. Asdiscussed in connection with the method of FIGS. 5A-5D, eventualincreases in temperature and pressure cause the voltage needed to ignitethe argon to increase above the voltage being applied by the RFgenerator, resulting in termination of conduction of the electrodes. Inthe FIG. 6A embodiment, the pressure of the argon can be maintaineddespite increases in temperature by withdrawing plunger 121 gradually asthe argon temperature increases. By maintaining the argon pressure, thethreshold voltage of the argon is also maintained, and so argon plasmawill continue to conduct current to the electrodes 132 a 132 b etc. Thismay be performed with or without moving the electrode 122.Alternatively, the position of electrode 122 may be changed during useso as to maintain a constant voltage threshold despite argon temperatureincreases.

FIGS. 7A and 7B show an alternative embodiment of an ablation device 210that is similar to the device of FIGS. 6A and 6B. In this embodiment,argon is sealed within reservoir 220 by a wall 217. Rather thanutilizing a plunger (such as plunger 121 in FIG. 6A) to change thevolume of reservoir 220, the FIGS. 7A-7B embodiment utilizes bellows 221formed into the sidewalls of housing 212. A pullwire 226 (which maydouble as the insulation for conductor 224) extends through internalelectrode 222 and is anchored to the distal end of the housing 212. Thebellows may be moved to the contracted position shown in FIG. 7A, theexpanded position shown in FIG. 7B, or any intermediate position betweenthem.

Pulling the pullwire 226 collapses the bellows into a contractedposition as shown in FIG. 7A and increases the pressure of the argonwithin the reservoir 220. Advancing the pullwire 226 expands the bellowsas shown in FIG. 7B, thereby decreasing the pressure of the argon. Thepullwire and bellows may be used to pre-select the threshold voltage,since (for a given temperature) increasing the argon pressure increasesthe threshold voltage of the argon and vice versa. Once the thresholdvoltage has been pre-set, operation is similar to that of the previousembodiments. It should be noted that in the third embodiment, thedistance between electrode 222 and ablation electrodes 232 a-c remainsfixed, although the device may be modified to allow the user to adjustthis distance and to provide an additional mechanism for adjusting thevoltage threshold of the device.

An added advantage of the embodiment of FIG. 7A is that the device maybe configured to permit the bellows 221 to expand in response toincreased argon pressure within the reservoir. This will maintain theargon pressure, and thus the threshold voltage of the argon, at a fairlyconstant level despite temperature increases within reservoir 220. Thus,argon plasma will continue to conduct current to the electrodes 132 a132 b etc and ablation may be continued, as it will be a longer periodof time until the threshold voltage of the argon exceeds the voltageapplied by the RF generator.

FIGS. 8A through 13B are a series of embodiments that also utilizeargon, but that maintain a fixed reservoir volume for the argon. In eachof these embodiments, current is conducted from an internal electrodewithin the argon reservoir to external ablation electrodes once thevoltage of the internal electrode reaches the threshold voltage of theargon gas.

Referring to FIGS. 8A and 8B, the fourth embodiment of an ablationdevice utilizes a housing 312 formed of insulating material, overlayinga conductive member 314. Housing 312 includes exposed regions 332 inwhich the insulating material is removed to expose the underlyingconductive member 314. An enclosed reservoir 320 within the housing 212contains argon gas, and an RF electrode member 322 is positioned withinthe reservoir. A return electrode (not shown) is attached to thepatient. The fourth embodiment operates in the manner described withrespect to FIGS. 5A-5D, except that the current returns to the RFgenerator via the return electrode on the patient's body rather than viaone on the device itself.

The fifth embodiment shown in FIGS. 9A and 9B is similar in structureand operation to the fourth embodiment. A conductive member 414 ispositioned beneath insulated housing 412, and openings in the housingexpose electrode regions 432 of the conductive member 414. The fifthembodiment differs from the fourth embodiment in that it is a bipolardevice having a return electrode 430 formed over the insulated housing412. Return electrode 430 is coupled to the RF generator and is cutawayin the same regions in which housing 412 is cutaway; so as to expose theunderlying conductor.

Internal electrode 422 is disposed within argon gas reservoir 420.During use, electrode regions 432 are placed into contact with bodytissue to be ablated. The RF generator is switched on and begins tobuild the voltage of electrode 422 relative to ablation electroderegions 432. As with the previous embodiments, conduction of ablationenergy from electrode 422 to electrode regions 432 will only begin onceelectrode 422 reaches the voltage threshold at which the argon inreservoir 420 ignites to form a plasma. Current passes through thetissue undergoing ablation and to the return electrode 430 on the deviceexterior.

The sixth embodiment shown in FIG. 10 is similar in structure andoperation to the fifth embodiment, and thus includes a conductive member514, an insulated housing 512 over the conductive member 512 and havingopenings to expose regions 532 of the conductive member. A returnelectrode 530 is formed over the housing 512, and an internal electrode522 is positioned within a reservoir 520 containing a fixed volume ofargon. The sixth embodiment differs from the fifth embodiment in thatthe exposed regions 532 of the conductive member 514 protrude throughthe housing 512 as shown. This is beneficial in that it improves contactbetween the exposed regions 532 and the target body tissue.

A seventh embodiment is shown in FIGS. 11A through 11C. As with thesixth embodiment, this embodiment includes an insulated housing 612(such as a heat resistant glass or ceramic) formed over a conductivemember 614, and openings in the insulated housing 612 to expose elevatedelectrode regions 632 of the conductive member 614. A return electrode630 is formed over the housing 612. An internal electrode 622 ispositioned within a reservoir 620 containing a fixed volume of argon.

The seventh embodiment differs from the sixth embodiment in that thereis an annular gap 633 between the insulated housing 612 and the elevatedregions 632 of the conductive member 614. Annular gap 633 is fluidlycoupled to a source of suction and/or to an irrigation supply. Duringuse, suction may be applied via gap 633 to remove ablation byproducts(e.g. tissue and other debris) and/or to improve electrode contact bydrawing tissue into the annular regions between electrode regions 632and ground electrode 630. An irrigation gas or fluid may also beintroduced via gap 633 during use so as to flush ablation byproductsfrom the device and to cool the ablation tip and the body tissue.Conductive or non-conductive fluid may be utilized periodically duringthe ablation procedure to flush the system.

Annular gap 633 may also be used to deliver argon gas into contact withthe electrodes 632. When the voltage of the electrode regions 632reaches the threshold of argon delivered through the gap 633, theresulting argon plasma will conduct from electrode regions 632 to theground electrode 630, causing lateral sparking between the electrodes632, 630. The resulting sparks create an “electrical file” which cutsthe surrounding body tissue.

An eighth embodiment of an ablation device is shown in FIGS. 12A and12B. This device 710 is similar to the device of the fifth embodiment,FIGS. 9A and 9B, in a number of ways. In particular, device 710 includesa conductive member 714 positioned beneath insulated housing 712, andopenings in the housing which expose electrode regions 732 of theconductive member 714. A return electrode 730 is formed over theinsulated housing 712. Internal electrode 722 is disposed within anargon gas reservoir 720 having a fixed volume.

The eighth embodiment additionally includes a pair of telescopingtubular jackets 740, 742. Inner jacket 740 has a lower insulatingsurface 744 and an upper conductive surface 746 that serves as a secondreturn electrode. Inner jacket 740 is longitudinally slidable betweenproximal position 740A and distal position 740B.

Outer jacket 742 is formed of insulating material and is slidablelongitudinally between position 742A and distal position 742B.

A first annular gap 748 is formed beneath inner jacket 740 and a secondannular gap 750 is formed between the inner and outer jackets 740, 742.These gaps may be used to deliver suction or irrigation to the ablationsite to remove ablation byproducts.

The eighth embodiment may be used in a variety of ways. As a firstexample, jackets 740, 742 may be moved distally to expose less than allof tip electrode assembly (i.e. the region at which the conductiveregions 732 are located). This allows the user to expose only enough ofthe conductive regions 732 as is needed to cover the area to be ablatedwithin the body.

Secondly, in the event bleeding occurs at the ablation site, returnelectrode surface 730 may be used as a large surface area coagulationelectrode, with return electrode surface 746 serving as the returnelectrode, so as to coagulate the tissue and to thus stop the bleeding.Outer jacket 742 may be moved proximally or distally to increase ordecrease the surface area of electrode 746. Moving it proximally has theeffect of reducing the energy density at the return electrode 746,allowing power to be increased to carry out the coagulation withoutincreasing thermal treatment effects at return electrode 746.

Alternatively, in the event coagulation and/or is needed, electrode 730may be used for surface coagulation in combination with a return patchplaced into contact with the patient.

FIGS. 13A-13B show a ninth embodiment of an ablation device utilizingprinciples of the present invention. The ninth embodiment includes aninsulated housing 812 having an argon gas reservoir 820 of fixed volume.A plurality of ablation electrodes 832 are embedded in the walls of thehousing 812 such that they are exposed to the argon in reservoir 832 andexposed on the exterior of the device for contact with body tissue. Areturn electrode 830 is formed over the housing 812, but includesopenings through which the electrodes 832 extend. An annular gap 833lies between return electrode 830 and housing 812. As with previousembodiments, suction and/or irrigation may be provided through the gap833. Additionally, argon gas may be introduced through the annular gap833 and into contact with the electrodes 832 and body tissue so as toallow argon gas ablation to be performed.

An internal electrode 822 is positioned within reservoir 820. Electrode822 is asymmetrical in shape, having a curved surface 822 a forming anarc of a circle and a pair of straight surfaces 822 b forming radii ofthe circle. As a result of its shape, the curved surface of theelectrode 820 is always closer to the electrodes 832 than the straightsurfaces. Naturally, other shapes that achieve this effect mayalternatively be utilized.

Electrode 822 is rotatable about a longitudinal axis and can also bemoved longitudinally as indicated by arrows in FIGS. 13A and 13B.Rotation and longitudinal movement can be carried out simultaneously orseparately. This allows the user to selectively position the surface 822a in proximity to a select group of the electrodes 832. For example,referring to FIGS. 13A and 13B, when electrode 822 is positioned asshown, curved surface 822 a is near electrodes 832 a, whereas no part ofthe electrode 822 is close to the other groups of electrodes 832 b-d.

As discussed earlier, the voltage threshold required to cause conductionbetween internal electrode 822 and ablation electrodes 832 will decreasewith a decrease in distance between the electrodes. Thus, there will bea lower threshold voltage between electrode 822 and the ablationelectrodes (e.g. electrode 832 a) adjacent to surface 822 a than thereis between the electrode 822 and ablation electrodes that are fartheraway (e.g. electrodes 832 b-d. The dimensions of the electrode 822 andthe voltage applied to electrode 822 are such that a plasma can only beestablished between the surface 822 a and the electrodes it is close to.Thus, for example, when surface 822 a is adjacent to electrodes 832 a asshown in the drawings, the voltage threshold between the electrodes 822a and 832 a is low enough that the voltage applied to electrode 822 willcause plasma conduction to electrodes 832 a. However, the thresholdbetween electrode 822 and the other electrodes 832 b-d will remain abovethe voltage applied to electrode 822, and so there will be no conductionto those electrodes.

This embodiment thus allows the user to selectively ablate regions oftissue by positioning the electrode surface 822 a close to electrodes incontact with the regions at which ablation is desired.

FIG. 14A shows a tenth embodiment of an ablation device utilizingvoltage threshold principles. The tenth embodiment includes a housing912 having a sealed distal end containing argon. Ablation electrodes 932a-c are positioned on the exterior of the housing 912. An internalelectrode 22 is disposed in the sealed distal end. Positioned betweenthe internal electrode 922 and the electrodes 932 a-c is a conductivegrid 933.

When electrode 922 is energized, there will be no conduction fromelectrode 922 to electrodes 932 a-c until the potential betweenelectrode 922 and the body tissue/fluid in contact with electrodes 932a-c reaches an initiating threshold voltage at which the argon gas willform a conductive plasma. The exact initiating threshold voltage isdependent on the argon pressure, its flowrate (if it is circulatingwithin the device), and the distance between electrode 922 and thetissue/body fluid in contact with the ablation electrodes 932 a-c.

Because the RF generator voltage output varies sinusoidally with time,there are phases along the RF generator output cycle at which the RFgenerator voltage will drop below the voltage threshold. However, oncethe plasma has been ignited, the presence of energized plasma ions inthe argon will maintain conduction even after the potential betweenelectrode 922 and the body fluid/tissue has been fallen below theinitiating threshold voltage. In other words, there is a thresholdsustaining voltage that is below the initiating threshold voltage, butthat will sustain plasma conduction.

In the embodiment of FIG. 14A, the grid 933 is spaced from theelectrodes 932 a-c by a distance at which the corresponding plasmaignition threshold is a suitable ablation voltage for the application towhich the ablation device is to be used. Moreover, the electrode 922 ispositioned such that once the plasma is ignited, grid 933 may bedeactivated and electrode 922 will continue to maintain a potentialequal to or above the sustaining voltage for the plasma. Thus, duringuse, both grid 933 and electrode 922 are initially activated for plasmaformation. Once the potential between grid 933 and body tissue/fluidreaches the threshold voltage and the plasma ignites, grid 933 will bedeactivated. Because ions are present in the plasma at this point,conduction will continue at the sustaining threshold voltage provided byelectrode 922.

The ability of ionized gas molecules in the argon to sustain conductioneven after the potential applied to the internal electrode has fallenbelow the initiating threshold voltage can be undesirable. As discussed,an important aspect of voltage threshold ablation is that it allows forhigh voltage/low current ablation. Using the embodiments describedherein, a voltage considered desirable for the application is selectedas the threshold voltage. Because the ablation electrodes are preventedfrom conducting when the voltage delivered by the RF generator is belowthe threshold voltage, there is no conduction to the ablation electrodeduring the rise time from 0V to the voltage threshold. Thus, there is noresistive heating of the tissue during the period in which the RFgenerator voltage is rising towards the threshold voltage.

Under ideal circumstances, conduction would discontinue during theperiods in which the RF generator voltage is below the threshold.However, since ionized gas remains in the argon reservoir, conductioncan continue at voltages below the threshold voltage. Referring to FIG.4A, this results in the sloping trailing edge of the ablation voltagewaveform, which approximates the trailing portion of the sinusoidalwaveform produced by the RF generator (FIG. 3). This low-voltageconduction to the tissue causes resistive heating of the tissue whenonly high voltage ablation is desired.

The grid embodiment of FIG. 14A may be used to counter the effect ofcontinued conduction so as to minimize collateral damage resulting fromtissue heating. During use of the grid embodiment, the trailing edge ofthe ablation voltage waveform is straightened by reversing the polarityof grid electrode 933 after the RF generator has reached its peakvoltage. This results in formation of a reverse field within the argon,which prevents the plasma flow of ions within the argon gas and thatthus greatly reduces conduction. This steepens the slop of the trailingedge of the ablation potential waveform, causing a more rapid droptowards 0V, such that it approximates the waveform shown in FIG. 4B.

FIGS. 15A and 15B show an eleventh embodiment utilizing principles ofthe present invention. As with the tenth embodiment, the eleventhembodiment is advantageous in that it utilizes a mechanism forsteepening the trailing edge of the ablation waveform, thus minimizingconduction during periods when the voltage is below the thresholdvoltage. In the eleventh embodiment, this is accomplished by circulatingthe argon gas through the device so as to continuously flush a portionof the ionized gas molecules away from the ablation electrodes.

The eleventh embodiment includes a housing 1012 having an ablationelectrodes 1032. An internal electrode 1022 is positioned within thehousing 1012 and is preferably formed of conductive hypotube havinginsulation 1033 formed over all but the distal-most region. A fluidlumen 1035 is formed in the hypotube and provides the conduit throughwhich argon flows into the distal region of housing 1012. Flowing argonexits the housing through the lumen in the housing 1012, as indicated byarrows in FIG. 15A. A pump 1031 drives the argon flow through thehousing.

It should be noted that different gases will have different thresholdvoltages when used under identical conditions. Thus, during use of thepresent invention the user may select a gas for the spark gap switchthat will have a desired threshold voltage. A single type of gas (e.g.argon) may be circulated through the system, or a plurality of gasesfrom sources 1033 a-c may be mixed by a mixer pump 1031a as shown inFIG. 15C, for circulation through the system and through the spark gapswitch 1035. Mixing of gases is desirable in that it allows a gasmixture to be created that has a threshold voltage corresponding to thedesired treatment voltage. In all of the systems using circulated gas,gas leaving the system may be recycled through, and/or exhausted from,the system after it makes a pass through the spark gap switch.

FIGS. 16A through 16D schematically illustrate the effect of circulatingthe argon gas through the device of FIG. 15A. Circulation preferably iscarried out at a rate of approximately 0.1 liters/minute to 0.8liters/minute.

Referring to FIG. 16A, during initial activation of the RF generator,the potential between internal electrode 1022 and ablation electrode1032 is insufficient to create an argon plasma. Argon molecules are thusnon-ionized, and the voltage measured at the load L is 0V. There is noconduction from electrode 1022 to electrode 1032 at this time.

FIG. 16B shows the load voltage measured from internal electrode 1022across the body fluid/tissue to return electrode 1030. Once the RFgenerator voltage output reaches voltage threshold V_(T) of the argon,argon molecules are ionized to create a plasma. A stream of the ionizedmolecules flows from electrode 1022 to electrode 1032 and current isconducted from electrode 1032 to the tissue. Because the argon isflowing, some of the ionized molecules are carried away. Nevertheless,because of the high voltage, the population of ionized molecules isincreasing at this point, and more than compensates for those that flowaway, causing an expanding plasma within the device.

After the RF generator voltage falls below V_(T), ion generation stops.Ionized molecules within the argon pool flow away as the argon iscirculated, and others of the ions die off. Thus, the plasma beginscollapsing and conduction to the ablation electrodes decreases andeventually stops. See FIGS. 16C and 16D. The process then repeats as theRF generator voltage approaches (−V_(T)) during the negative phase ofits sinusoidal cycle.

Circulating the argon minimizes the number of ionized molecules thatremain in the space between electrode 1022 and electrode 1032. If a highpopulation of ionized molecules remained in this region of the device,their presence would result in conduction throughout the cycle, and thevoltage at the tissue/fluid load L would eventually resemble thesinusoidal output of the RF generator. This continuous conduction at lowvoltages would result in collateral heating of the tissue.

Naturally, the speed with which ionized molecules are carried awayincreases with increased argon flow rate. For this reason, there will bemore straightening of the trailing edge of the ablation waveform withhigher argon flow rates than with lower argon flow rates. This isillustrated graphically in FIG. 17. The upper waveform shows the RFgenerator output voltage. The center waveform is the voltage outputmeasured across the load (i.e. from the external electrode 1032 acrossthe body tissue/fluid to the return electrode 1030) for a device inwhich the argon gas is slowly circulated. The lower waveform is thevoltage output measured across the load for a device in which the argongas is rapidly circulated. It is evident from the FIG. 17 graphs thatthe sloped trailing edge of the ablation waveform remains when the argonis circulated at a relatively low flow rate, whereas the trailing edgefalls off more steeply when a relatively high flow rate is utilized.This steep trailing edge corresponds to minimized current conductionduring low voltage phases. Flow rates that achieve the maximum benefitof straightening the trailing edge of the waveform are preferable. Itshould be noted that flow rates that are too high can interfere withconduction by flushing too many ionized molecules away during phases ofthe cycle when the output is at the threshold voltage. Optimal flowrates will depend on other physical characteristics of the device, suchas the spark gap distance and electrode arrangement.

It should also be noted that the distance between internal electrode1022 and external electrode 1032 also has an effect on the trailing edgeof the ablation potential waveform. In the graphs of FIG. 18, the RFgenerator output is shown in the upper graph. V_(PRFG) represents thepeak voltage output of the RF generator, V_(T1) represents the voltagethreshold of a device having a large separation distance (e.g.approximately 1 mm) between electrodes 1022 and 1032, and V_(T2)represents the voltage threshold of a device in which electrodes 1022,1032 are closely spaced—e.g. by a distance of approximately 0.1 mm. Aspreviously explained, there is a higher voltage threshold in a devicewith a larger separation distance between the electrodes. This isbecause there is a large population of argon molecules between theelectrodes 1022, 1032 that must be stripped of electrons before plasmaconduction will occur. Conversely, when the separation distance betweenelectrodes 1022 and 1032 is small, there is a smaller population ofargon molecules between them, and so less energy is needed to ionize themolecules to create plasma conduction.

When the RF generator output falls below the threshold voltage, themolecules begin to deionize. When there are fewer ionized molecules tobegin with, as is the case in configurations having a small electrodeseparation distance, the load voltage is more sensitive to thedeionization of molecules, and so the trailing edge of the outputwaveform falls steeply during this phase of the cycle.

For applications in which a low voltage threshold is desirable, thedevice may be configured to have a small electrode spacing (e.g. in therange of 0.001-5 mm, most preferably 0.05-0.5 mm) and non-circulatingargon. As discussed, doing so can produce a load output waveform havinga steep rising edge and a steep falling edge, both of which aredesirable characteristics. If a higher voltage threshold is needed,circulating the argon in a device with close inter-electrode spacingwill increase the voltage threshold by increasing the pressure of theargon. This will yield a highly dense population of charged ions duringthe phase of the cycle when the RF generator voltage is above thethreshold voltage, but the high flow rate will quickly wash many ionsaway, causing a steep decline in the output waveform during the phasesof the cycle when the RF generator voltage is below the threshold.

A twelfth embodiment of a system utilizing principles of the presentinvention is shown schematically in FIG. 19. The twelfth embodimentallows the threshold voltage to be adjusted by permitting the spark gapspacing (i.e. the effective spacing between the internal electrode andthe ablation electrode) to be selected. It utilizes a gas-filled sparkgap switch 1135 having a plurality of internal spark gap electrodes 1122a, 1122 b, 1122 c. Each internal electrode is spaced from ablationelectrode 1132 by a different distance, D1, D2, D3, respectively. Anadjustment switch 1125 allows the user to select which of the internalelectrodes 1122 a, 1122 b, 1122 c to utilize during a procedure. Sincethe threshold voltage of a spark gap switch will vary with the distancebetween the internal electrode and the contact electrode, the user willselect an internal electrode, which will set the spark gap switch tohave the desired threshold voltage. If a higher threshold voltage isused, electrode 1122 a will be utilized, so that the larger spark gapspacing D1 will give a higher threshold voltage. Conversely, the userwill selected electrode 1122 c, with the smaller spark gap spacing, if alower threshold voltage is needed.

It is useful to mention that while the spark gap switch has beenprimarily described as being positioned within the ablation device, itshould be noted that spark gap switches may be positioned elsewherewithin the system without departing with the scope of the presentinvention. For example, referring to FIG. 19, the spark gap switch 1135may be configured such that the ablation electrode 1132 disposed withinthe spark gap is the remote proximal end of a conductive wire that iselectrically coupled to the actual patient contact portion of theablation electrode positioned into contact with body tissue. A spark gapswitch of this type may be located in the RF generator, in the handle ofthe ablation device, or in the conductors extending between the RFgenerator and the ablation device.

Several embodiments of voltage threshold ablation systems, and methodsof using them, have been described herein. It should be understood thatthese embodiments are described only by way of example and are notintended to limit the scope of the present invention. Modifications tothese embodiments may be made without departing from the scope of thepresent invention, and features and steps described in connection withsome of the embodiments may be combined with features described inothers of the embodiments. Moreover, while the embodiments discuss theuse of the devices and methods for tissue ablation, it should beappreciated that other electrosurgical procedures such as cutting andcoagulation may be performed using the disclosed devices and methods. Itis intended that the scope of the invention is to be construed by thelanguage of the appended claims, rather than by the details of thedisclosed embodiments.

We claim:
 1. A method of treating tissue using an electrosurgicalsystem, comprising the steps of: (a) providing an electrosurgical systemhaving an RF generator, a treatment electrode electrically coupled tothe RF generator and positioned in contact with target tissue to betreated, and a spark gap switch positioned between the RF generator andthe target tissue, the spark gap having a threshold voltage to preventconduction of current from the RF generator to the tissue until thevoltage across the spark gap reaches the threshold voltage; and (b)using the RF generator, applying a voltage across the spark gap switch,the spark gap switch causing conduction of current from the RF generatorto the target tissue once the voltage across the spark gap reaches thethreshold voltage.
 2. The method of claim 1 wherein step (a) providesthe spark gap switch to include a conductor, wherein the spark gapextends between the conductor and the treatment electrode.
 3. The methodof claim 1, wherein step (a) provides a gas within the spark gap.
 4. Themethod of claim 3 wherein step (a) provides the gas to be a containedvolume of gas.
 5. The method of claim 3, further including the step offlowing the gas through the spark gap.
 6. The method of claim 5, whereinthe flowing step includes flowing the gas through the spark gap at aconstant rate.
 7. The method of claim 5, wherein the flowing stepincludes varying the rate at which the gas flows through the spark gap.8. The method of claim 1, further including the step of adjusting thethreshold voltage of the spark gap switch.
 9. The method of claim 8,wherein step (a) provides a gas within the spark gap, and wherein theadjusting step includes changing the pressure of the gas in the sparkgap.
 10. The method of claim 8 wherein the adjusting step includeschanging the length of the spark gap.
 11. The method of claim 10 whereinstep (a) provides the spark gap switch to include a conductorelectrically coupled to the RF generator, the spark gap extendingbetween the conductor and the treatment electrode, wherein the adjustingstep includes changing the distance between the conductor and thetreatment electrode.
 12. The method of claim 11, wherein the step ofchanging the distance includes moving the conductor and/or the treatmentelectrode relative to the other of the conductor and/or the treatmentelectrode.
 13. The method of claim 8 wherein: step (a) provides aplurality of spaced apart spark gap electrodes wherein each pair of theelectrodes defines a spark gap having a threshold voltage, differentpairs of the conductors defining spark gaps having different thresholdvoltages; the adjusting step includes selecting a pair of conductorsdefining a spark gap having the desired threshold voltage; and step (b)includes applying the voltage across the selected pair of theconductors, causing conduction of current from the RF generator to thetarget tissue to occur once the voltage across the selected pair of theconductors reaches the desired threshold voltage.
 14. The method ofclaim 5, further including the step of adjusting the threshold voltageby changing the rate of gas flow in the spark gap.
 15. The method ofclaim 3 further including the step of adjusting the threshold voltage bychanging the composition of the gas in the spark gap.
 16. The method ofclaim 3 further including the step of adjusting the threshold voltage bychanging the temperature of the gas in the spark gap.
 17. The method ofclaim 3 further including the step of controlling the temperature of thetarget tissue by controlling the pressure of the gas in the spark gap.18. The method of claim 17 wherein the controlling step includesconfiguring the spark gap such that an increase in pressure above athreshold level increases the threshold voltage of the spark gap to beabove the voltage being applied across the spark gap switch, and causesflow of current from the RF generator to the tissue to automaticallyterminate.
 19. The method of claim 3 further including the step ofcontrolling the temperature of the target tissue by controlling thevolume of the gas in the spark gap.
 20. The method of claim 19 whereinstep (a) provides the spark gap to be expandable in response toincreased gas pressure within the spark gap, and wherein the step ofcontrolling the volume includes allowing the volume of gas in the sparkgap to expand in response to increased pressure in the spark gap, saidexpansion increasing the size of the spark gap and increasing thethreshold voltage of the spark gap to be above the voltage appliedacross the spark gap switch, causing flow of current from the RFgenerator to the tissue to automatically terminate.
 21. The method ofclaim 19 wherein step (a) provides the spark gap to be expandable inresponse to increased gas pressure within the spark gap, and wherein thestep of controlling the volume includes allowing the volume of gas inthe spark gap to expand in response to increased pressure in the sparkgap while maintaining the spacing of the spark gap electrodes.
 22. Themethod of claim 1 wherein step (a) provides a control electrode disposedwithin the spark gap, and wherein the method includes applying apotential to the control electrode to initiate flow of current from theRF generator across the spark gap to the target tissue.
 23. The methodof claim 1 wherein step (a) provides a control electrode disposed withinthe spark gap, and wherein the method includes applying a potential tothe control electrode to terminate flow of current from the RF generatorto the tissue.
 24. The method of claim 1 wherein step (a) provides acontrol electrode disposed within the spark gap, and wherein the methodincludes applying a first potential to the control electrode to initiateflow of current from the RF generator to the tissue and applying asecond potential to the control electrode to terminate flow of currentfrom the RF generator to the tissue, the first and second potentialsbeing of opposite polarities.
 25. The method of claim 9 wherein the stepof changing the pressure of the gas in the spark gap includes the stepof changing the volume of the gas reservoir.
 26. A method of treatingtissue using an electrosurgical instrument, comprising the steps of:providing an electrosurgical instrument having a body, an enclosedreservoir within the body and containing a gas, a conductor disposedwithin the gas, and an electrode spaced from the conductor, theelectrode having an interior portion exposed to the gas and an exteriorportion on an exterior of the body; placing the exterior portion of theelectrode into contact with tissue to be treated; applyingelectrosurgical energy to the conductor, the gas initially insulatingthe conductor from the interior portion of the electrode; continuing toapply electrosurgical energy to the conductor, to cause the gas to forma conductive plasma, the plasma conducting the electrical energy fromthe conductor to the electrode, causing electrical energy to beconducted from the electrode to the tissue.
 27. The method of claim 26wherein the gas in the reservoir has a voltage threshold at which theplasma will form, and wherein the continuing step includes applyingelectrosurgical energy at the voltage threshold to cause the plasma toform.
 28. The method of claim 27, including the step of changing thevoltage threshold of the gas.
 29. The method of claim 28, wherein thechanging step includes changing the volume of the reservoir.
 30. Themethod of claim 28, wherein the changing step includes changing thepressure of the gas.
 31. The method of claim 28, wherein the methodincludes circulating the gas and wherein the changing step includeschanging the circulation rate of the gas.
 32. The method of claim 28,wherein the conductor is moveable relative to the interior portion ofthe electrode, and wherein the changing step includes changing thedistance between the conductor and the interior portion of theelectrode.
 33. The method of claim 26 wherein: the providing stepprovides a plurality of electrodes, each having an interior portionexposed to the gas and an exterior portion, in the placing step at leasta portion of the plurality of electrodes is placed into contact withtissue to be treated; in the applying step, the gas initially insulatesthe conductor from the electrodes; and the continuing step causes theplasma to conduct electrical energy from the conductor to at least aportion of the electrodes.
 34. The method of claim 33 wherein: theconductor provided in the providing step includes a rotatable internalelectrode having a laterally extending portion moveable into positionadjacent to select pluralities of the electrodes; the method furtherincludes the step of rotating the internal electrode to orient thelaterally extending portion adjacent to a select plurality of theelectrodes; and in the continuing step a plasma is formed between thelaterally extending portion of the internal electrode and the selectplurality of the electrodes to cause conduction of electrical energythrough the select plurality of the electrodes and into the tissue. 35.The method of claim 26 wherein the providing step provides the reservoirto contain argon gas.
 36. The method of claim 26, further including thestep of circulating the gas within the body.
 37. The method of claim 26wherein: the electrosurgical device is further provided to include areturn electrode on its exterior, and a slidable insulating cover overthe return electrode; and the method includes the step of withdrawingthe insulating cover relative to the return electrode to increase thesurface area of the return electrode relative to the surface area of theelectrode.
 38. The method of claim 26 wherein: the electrosurgicaldevice is further provided to include a return electrode on itsexterior, and a slidable insulating cover over the return electrode; andthe method includes the step of advancing the insulating cover relativeto the return electrode to decrease the surface area of the returnelectrode relative to the surface area of the electrode.
 39. The methodof claim 26 wherein the electrosurgical device is further provided toinclude a secondary conductor disposed within the reservoir, and whereinthe method includes the steps of: during the continuing step, applying apotential to the secondary conductor until the plasma has been formedand then discontinuing application of potential to the secondaryconductor.
 40. The method of claim 39 wherein the gas in the reservoirhas a voltage threshold at which the plasma will form, wherein thecontinuing step includes applying electrosurgical energy to theconductor and the secondary conductor to cause the voltage across thereservoir to reach the voltage threshold and to cause the plasma toform, wherein the gas has a sustaining threshold voltage at which aplasma will be sustained once it has been initiated, and wherein thestep of discontinuing application of potential to the secondaryconductor causes the voltage across the reservoir to remain above thesustaining voltage.